1. Field of the Invention
The subject invention relates to the portion of a medication delivery pen that retains the cartridge of medication.
2. Description of the Prior Art
Medication delivery pens are hypodermic syringes that are used for self-injection of precisely measured doses of medication. Pens are widely used, for example, by diabetics to dispense insulin.
The typical prior art medication delivery pen includes a cartridge which contains a volume of liquid medication sufficient for several doses. The prior art cartridge includes an elongated generally tubular glass vial having an open proximal end and an opposed open distal end. The vial includes a large diameter barrel extending distally from the open proximal end to an inwardly tapering shoulder between the two ends. A short small diameter neck extends from the shoulder to the open distal end. The neck of the prior art vial has an annular rim projecting outwardly around the extreme distal end.
The prior art cartridge further includes a pierceable rubber septum which extends across the open distal end of the prior art vial, and is securely held in position by a metallic sleeve that is crimped to the annular rim on the tubular neck. The vial of the prior art cartridge is filled with liquid medication, and a rubber stopper is inserted into the open proximal end of the vial for sliding fluid-tight engagement with interior walls of the barrel.
The prior art medication delivery pen includes a unitarily molded cartridge retainer with opposed proximal and distal ends. A large diameter tubular body extends distally from the proximal end and is dimensioned for receiving the barrel of the vial. A short smaller diameter tubular neck is disposed distally of the body and is dimensioned for tightly engaging the tubular neck of the vial and the metallic sleeve crimped thereon so as to support and position the entire cartridge. Exterior regions at the extreme distal end of the tubular neck are formed with an array of threads for threadedly receiving the mounting cap of a needle assembly.
The prior art medication delivery pen further includes a dosing apparatus that is engaged with the proximal end of the cartridge retainer. The prior art dosing apparatus includes a plunger for engaging the rubber stopper of the cartridge, dose setting structure for selecting the longitudinal distance through which the plunger will move, and dispensing means for driving the plunger the selected distance. The prior art dosing apparatus may be permanently connected to prior art cartridge retainer with the cartridge therein. This type of prior art pen is used until the medication is exhausted and then the entire pen is discarded. Other prior art medication delivery pens may have the dosing apparatus removably connected to the cartridge retainer so that at least portions of the pen may be reused when the medication in the cartridge is exhausted.
Prior art needle assemblies for medication delivery pens are safely sealed in packages. A needle assembly is accessed immediately prior to an injection, and is discarded immediately after the injection. The prior art needle assembly for medication delivery pens includes an elongate needle cannula having opposed proximal and distal points and a lumen extending therethrough. A plastic cork is adhered to an intermediate position along the needle cannula and in turn is rigidly connected to an end wall of a cylindrical cap. The cylindrical wall of the cap surrounds the proximal point on the needle cannula and includes an array of internal threads for engaging the external threads on the neck of the prior art cartridge retainer.
The prior art medication delivery pen may be used by opening the sealed needle assembly and urging the cap over the neck of the vial retainer sufficiently for the proximal point of the needle cannula to pierce the rubber septum of the prior art cartridge. The cap is then rotated to threadedly engage the neck of the prior art cartridge retainer. The user will then manipulate the dosing apparatus to select an appropriate dose. A protective shield over the distal end of the needle cannula is then removed, and the distal point of the needle cannula is injected. The user then actuates the dispensing means of the prior art dosing apparatus to urge the stopper of the cartridge distally and to deliver medication through the lumen of the needle cannula. The needle is then withdrawn, and the needle assembly is separated from the cartridge retainer and safely discarded. The rubber septum of the cartridge will reseal itself, and may be pierced again for a subsequent administration of medication. This process may be carried out repeatedly until all of the medication in the cartridge has been used.
The neck at the distal end of the prior art unitarily molded cartridge retainer has been precisely formed to closely engage, support and position the entire cartridge. However, cartridges are subject to a considerable range of dimensional variations and a considerable degree of eccentricity. These dimensional variations and eccentricities may be due to the glass vial manufacturing processes or to the crimping of the metallic sleeve that holds the rubber septum in place. Dimensional variations can result in a cartridge that will not fit the cartridge retainer or that will be loosely supported and movable therein. Eccentricities can result in a vial barrel that is not properly positioned or aligned within the body of the cartridge retainer. Eccentricities also can prevent the neck of the vial from sliding into the precisely dimensioned neck of the cartridge retainer. As a result, considerable quality control efforts must be undertaken to ensure that only cartridges that are within narrowly defined dimensional tolerances are used, and high reject rates occur. To reduce rejects and ensure that a larger number of vials can be accepted, prior art pens have included vial retainers with wider bodies that are intended to accommodate a greater range of eccentricities between the neck and the body of the vial. This results in larger pens even though it would be desirable to reduce the size.
Users of medication delivery pens are urged to disinfect both the puncture site and the distal end of the pen prior to each injection of medication. The disinfectant can react with the plastic of the prior art cartridge retainer to cause crazing or cracking.
Medication delivery pens also have been found to exhibit weeping or drooling near the interface of the needle assembly and the cartridge retainer. This weeping or drooling presents inconveniences to the user and creates the potential for an accumulation of medication at an external position on the pen and near the puncture site of the patient.
It is now believed that weeping or drooling is attributable to contact between the septum and the cork of the needle assembly during the injection of medication. In particular, the movement of the plunger distally in the vial urges the liquid in a distal direction. These distally directed forces urge liquid through the lumen of the needle cannula. However, these forces also cause a stretching of the septum in a distal direction. As noted above, the neck of the prior art cartridge retainer provides the primary support for the cartridge, and hence closely engages the metal sleeve which holds the septum to the vial. The distal stretching of the septum in response to fluid pressure urges the septum into direct contact with the cork on the needle assembly of the prior art medication delivery pen. The combination of fluid pressure and contact with the cork will sufficiently change the shape of the pierced septum to permit the weeping or drooling of medication between the septum and the needle cannula.
Still another problem with prior art medication delivery pens relates to the above described disposition of threads around the distal end of the prior art cartridge retainer. In particular, the threads begin at the distal tip of the prior art cartridge retainer and extend a short distance in a proximal direction. Even minor variations in dimensional tolerances can require the user to threadedly engage the cap of the needle assembly to the threads of the cartridge retainer before the proximal tip of the needle cannula has pierced the septum. In these instances, the beveled proximal tip of the needle cannula will pierce the septum while undergoing a rotational movement. This may cause the beveled tip to rip the septum and may further contribute to the above described drooling or weeping. Sufficiently large rips may not adequately reseal and can lead to a premature degradation of the medication stored in the vial.